1. Field of the Invention
The present invention relates to a novel zoom lens and, in particular, to a zoom lens suitable for a video camera or a digital still camera, and a camera system using the same. Specifically, this invention relates to a camera system covering an ultra-wide-angle range and having a small F-number at a wide angle end, in which various aberrations other than distortion are corrected properly. Moreover, video signals obtained from a zoom lens and an image sensor, having an extremely small front lens diameter, are processed to correct the distortion due to this zoom lens, thereby obtaining an excellent image.
2. Description of Related Art
Japanese Examined Patent Application Publication (KOKOKU) No. Sho 50-26931 (Patent document 1) indicates that it is suitable for achieving high power to employ a five-group zoom system constructed by arranging refracting power in order from an object side: positive; negative; positive; negative; and positive, in which a first lens group, a third lens group, and a fifth lens group are stationary, a second lens group is shifted in an optical axis direction to perform variable power, and a fourth lens group corrects a focal shift due to zooming.
While the zoom ratio in the above patent document 1 is approximately 20 times, the applicant has proposed in Japanese Patent Application Publication (KOKAI) No. Hei 11-84239 (Patent document 2) an ultrahigh ratio zoom lens, small in size but having a zoom ratio reaching 50 times, by developing the above five-group zoom system. Specifically, in this zoom system, the fourth lens group also performs focusing, an aspheric surface is introduced into the first lens group, and the shapes of the lenses in the respective lens groups are optimized while reducing the number of the lenses.
Moreover, the applicant has proposed in Japanese Patent Application Publication (KOKAI) No. 2000-105336 (Patent document 3) an ultra-wide-angle, ultrahigh ratio zoom lens whose angle of view at a wide angle end reaches approximately 86° and whose zoom ratio is approximately 40 times, by applying a lens configuration suitable for ultra-wide-angle to the first lens group in the above five-group zoom system.
Since a high power zoom lens involves an increase in total length, inconvenience may occur depending on the shape of the camera system. Consequently, the applicant has proposed in Japanese Patent Application Publication (KOKAI) No. Hei 8-248318 (Patent document 4) means that shorten the dimension, in a depth direction, by disposing a concave lens group at a position closest to an object side in the first lens group, and disposing a right-angle prism immediately behind it so as to refract an optical path.